Integral skin foams are well known to those skilled in the art of polyurethane foams. Such foams have a cellular interior and a higher density microcellular or non-cellular skin. In general, to prepare such foams an organic isocyanate is reacted with a substance having at least one isocyanate reactive group in the presence of a catalyst, blowing agent, and a variety of optional additives. The reaction is carried out in a mold where a higher density skin forms at the interface of the reaction mixture and the relatively cool inner surface of the foam.
Historically, the most common types of blowing agent used in integral skin polyurethane foams have been chlorofluorocarbons (CFCs) or combinations of CFCs and other blowing agents. However, in view of recent mandates calling for a reduction and eventually elimination of the use of CFCs, alternatives are considered necessary.
Past methods of preparing integral skin polyurethanes with CFCs as a blowing agent includes G.B. Patent No. 1,209,297, which teaches the use of a combination blowing agent consisting of a CFC and hydrate of an organic compound which splits off water at temperatures above 40.degree. C. This blowing agent or combination of agents was used in a formulation with a suitable polyisocyanate, a polyol containing hydroxyl group and a catalyst. This patent discloses that free water in the system leads to a skin that is permeated with fine cells, which is undesirable.
Attempts have been made to evaluate the performance of alternate blowing agents to CFCs. In a paper by J. L. R. Clatty and S. J. Harasin entitled, Performance of Alternate Blowing Agents to Chlorofluorocarbons in RIM Structural and Elastomeric Polyurethane Foams, presented to the Annual Polyurethane Technical/Marketing Conference, October 1989, the authors addressed the use of water as a blowing agent for integral skin polyurethane reaction injection molded systems (RIM). In this application, the water concentration in the system is controlled by the concentration and type of molecular sieves used. As in the Great Britain patent discussed previously, the water is not in a free form but bound in some manner. In this instance, the authors state that this process is limited to use in rigid foam systems; and the flexible integral skin formulations may best be served by using HCFCs or HCFC-22 as substitutes for CFCs.
A recently employed integral skin foam formulation is described in U.S. Pat. No. 5,100,922 to Wada et al. which relates to a method for producing a molded product of integral skin polyurethane foam. The method comprises reacting and curing (1) a high molecular weight polyol comprising, as the main component, a polyoxyalkylene polyol having, as the main constituent, oxyalkylene groups of at least 3 carbon atoms and oxyethylene groups at its molecular terminals with the overall oxyethylene group content being not higher than 15% by weight and having a hydroxyl value of not higher than 80, (2) a crosslinking agent containing a compound having an aromatic nucleus and at least two active hydrogen containing groups selected from the group consisting of hydroxyl groups, primary amino groups and secondary amino groups, and (3) a polyisocyanate, in a mold in the presence of a catalyst and a hydrogen atom containing halogenated hydrocarbon foaming agent. While an extensive list of blowing agents are provided, the only pentafluoro compounds described are chlorinated compounds such as 3,3-dichloro-1,1,1,2,2-pentafluoropropane and 1,3-dichloro-1,1,2,2,3-pentafluoropropane, which are considered undesirable.
More recently U.S. Pat. No. 5,506,275, issued to Valoppi, the present inventor, which relates to the use of 1,1,1,2-tetrafluoroethane as an alternative to conventional chlorinated fluorocarbon blowing agents in integral skin foam formulations. While this patent offers an alternative to halogenated hydrocarbon blowing agents per se, 1,1,2-tetrafluoroethane (HFC-134a) boils at -26.5.degree. C. and thus requires special gas delivery systems to introduce and maintain the blowing agent in solution, especially in warm weather conditions, i.e., above 90.degree. F. As such, still further improvements in the art are considered necessary.
It has been found that foams utilizing pentafluoropropane blowing agents and, in particular, 1,1,1,3,3-pentafluoropropane as the blowing agent alone or in combination with limited amounts of water can be prepared which meet the stringent requirements inherent to integral skin foam applications such as an acceptable appearance and must exhibit enhanced resistance to abrasion and cracking upon flex. Further, the pentafluoropropane blowing agents utilized in association with the present invention are generally soluble in resinous solution thus eliminating or greatly reducing the need for specialized gas delivery systems to maintain pressure on the system.